The present invention relates to signalling apparatus and more particularly to an intrusion or theft alarm in which a self-powered sensor/transmitter signals to a remote receiver.
A great many types of burgular alarms have been developed for detecting intrusion into a protected space. In such systems where each possible entry point, window or door, is individually sensed as to opening and closing, the installation of the system has typically required that connecting wires be strung to each sensor. The cost of installing such wiring is very high and will typically even exceed the cost of the physical components of the system. While some so-called wireless systems are in use, they in fact require some source of electrical power.
While some theft detection systems utilize self-powered sensors which do not require wiring, the sensors do require the presence of an energizing r.f. field. These sensors typically operate by disturbing the applied r.f. field or utilizing received energy to re-transmit on a different frequency.
Among the several objects of the present invention may be noted the provision of apparatus for signalling remotely a sensed change in a local condition; the provision of such a system in which the sensor units do not require hard wired connection to the remote location; the provision of such a system in which the individual sensors do not require a separate electrical power source such as a battery; the provision of an improved intrusion or theft detector; the provision of such apparatus which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.